Electrical resistor and the like



Dec. 20, 1966 ROBINSON ELECTRICAL RESISTOR AND THE LIKE Original FiledMarch 4, 1963 ower Source INVENTOR ATTORNEYS 3,293,587 ELECTRICALRESISTOR AND THE LIKE Preston Robinson, Wiliiamstown, Mass, assignor toSprague Electric Company, North Adams, Mass,

a corporation of Massachusetts Continuation of abandoned applicationSer. No. 262,626, Mar. 4, 1963. This application Oct. 20, 1965, Ser. No.506,426

2 Claims. (Cl. 338-400) The present application is a continuation ofapplication Serial No. 262,626, filed March 4, 1963, now abandoned whichin turn is a continuation-in-part of application Serial No. 807,247,filed April 17, 1959, now US. Patent No. 3,080,481, issued March 5,1963. The last-mentioned application is a continuation-in-part ofapplication Serial No. 553,893, filed December 19, 1955, now US. Patent2,883,554, granted April 21, 1959.

This invention relates to surface film type resistor devices and thelike, and to the apparatus and method for making them.

There are various types of surface film devices including resistors andinductances to which this invention may be applied. Among such types arethe resistors of the metal film type in which the resistor elementcomprises a ceramic base which has been coated with a thin film ofconductive metal and spiraled with a non-conducting strip to provide thefinal resistance. The invention may also be applied to an inductancewherein a conducting material is coated on an elongated base and theconducting turns are produced by cutting a helical path between turns ofthe conductive area.

Rapid production of resistors and other semiconductive devices andinductances presents problems in reproducibility, accuracy andflexibility. Resistances in such devices have tended to show variancesfrom specification, and rejected units resulting from mechanical damageto the resistance element during production are undesirable. Thetemperature coeflicients of resistance and of expansion in the surfacefilm type of resistor affect the stability thereof. Therefore, inelectrical circuits requiring a high stability of resistance values overa wide temperature range, a surface film type resistor produced by rapidproduction may vary from specified limitations because of the mechanicaldamage to the conducting film.

One of the problems encountered in producing a spiraled surface filmtype device is the time and effort involved in creating the insulatinggroove or cut which produces the lengthened conductive element of thedevice. The conventional means for producing this nonconducting spiralgroove is a grinding operation using a sharp Carborundum grinding toolwhich knifes into the substrate through the surface film. The speed offorming the non-conducting groove in this manner is limited andconsequently the integration of the production of surface film typedevices into automated production is hindered. Moreover, the grindingtechnique makes an incision into the substrate to a considerable depthin order to surely and effectively provide the necessary separationbetween the turns of the conducting material. This deep incisionintroduces into the resultant device objectionable features which mustbe overcome. For example, the opening made in the device by the grindingwill have a tendency to collect contaminants unless this is compensatedfor.

ited States Patent F Patented Dec. 20, 1966 An important feature of thesurface film type of resistor is stable operation at a high wattagerating. This requires an effective dissipation of heat equally along thewhole length of the resistor. Where the dissipation of the heat isuneven and the temperature gradient along the surface of the resistor ishigh, there is a detrimental effect on the stability of the resistor.Moreover, it is desirable to provide the highest possible wattage ratingfor a given hot spot temperature in a resistor.

It is an object of this invention to provide a resistor 0r semiconductordevice by rapidly forming non-conducting portions between conductiveturns.

It is another object of this invention to provide a technique for thepreparation of resistor or semiconductor bodies in a simple manner whichaccurately reproduces identical completed devices.

It is a further object of this invention to provide an electricalresistor having a rapid and even dissipation of heat.

Still another object of this invention is to provide a resistor havinggood electrical stability over a wide temperature range.

A still further object of this invention is a surface film type resistorwith a helical groove having lower temperature coefficient of resistancethan previously available.

Another object of this invention is to provide rapid helixing of asurface film type device to permit integration of the helixing in anautomated process.

Still another object of this invention is the removal of a portion ofthe conductive film in a surface film type device without cuttingsubstantially into the substrate.

These and other objects of this invention will become more apparent uponconsideration of the following description taken together with theaccompanying drawing in which:

FIG. 1 is an enlarged scale axial sectional view showing a constructionin accordance with the invention;

FIG. 2 is a fragmentary axial sectional view of a portion of the surfaceof the construction of FIG. 1; and

FIG. 3 is a diagrammati section of apparatus for making a device of thisinvention.

The resistors of the present invention are coated with a layer ofresistive or semiconductive material deposited on an elongatednon-conducting base. The conducting layer is formed into a conductingtrack by a helical insulating path formed around the non-conductingbase. The helical non-conducting path is produced on the device throughthe action of a focused beam of electrons on the coated base to causethe formation of the lengthened resistive path ofthe conducting track.

A surface film type device as referred to herein includes metal filmresistors in which a non-conducting base such as a ceramic rod is coatedwith a metal film and thereafter a nonconducting strip is formed bytaking away part of the conducting coat in a helical path around therod. This results in lengthening the resistive path of the remainingconducting material. The conductive material has a resistivity and thisresistivity provides the resistance value of the ultimate product.Conductivity and resistivity as used in reference to the surface filmtype device are relative terms. For example, resistivities can rangefrom below 10- ohrn-cm. to about 10 ohm-cm. for silicon, illustratingthat in principle a wide range of resistances are possible for thischemical element. Moreover, a satisfactory resistor can be produced with1 ohm-cm.

silicon. It is thus seen that semiconductivity and resistivity havesimilar connotations.

Referring to FIG. 1, there is shown a resistor unit of this invention inwhich an insulation core 11 is shown with a peripheral metal filmelement 21 in spiral convolution on a surface 13 of the core 11. Thehelical shape is produced by removing the resistive or semiconductivematerial of film 12 in the manner of thread cutting. A non-conductingpath 14 is the result of helixing and separates turns of the film 12 andforms a portion of the film 12 into a conducting track 15. The film 12is originally deposited on the surface 13 of the ceramic core 11 towhich it bonds firmly. End terminations 16 are of noble metal in orderto insure low contact resistance with the resistance film 12. Metal endcaps 17 and leads 18 are provided at either end. The end caps 17 are incontact with the end terminations 16.

The enlarged view of FIG. 2 serves to illustrate the insulation of thesuccessive turns of the conducting track by the intervening turns of thenon-conducting path.

The steps of providing the helixed resistor 10 comprise first coatingthe ceramic core 11 with the resistive film 12 and thereafter helixingthe coated core 11 by evaporating the non-conducting path 14 under theimpingement of an electron beam. The resultant evaporation rapidlyremoves the material of film 12 as the resistor 10 is rotated on itsaxis while the electron beam is scanned along the length of resistor 10.This results in a lengthening of the resistive path and hence anincrease in resistance value. The core is initially coated to give aninitial value Ri and and thereafter the helical non-conducting path isformed extending along the length of the resistor 10 until the desiredfinal resistance value Rf is attained. The helixing process is carriedout automatically, the resistor being rotated on its axis While at thesame time the electron beam is advanced at a constant speed along thelength of the resistor 10. The increase in resistance is measured as itoccurs by integral circuitry and means are provided for detecting theattainment of a given value and for instant cessation of the helixing.

The apparatus for treatment of the resistor is shown in FIG. 3. A vacuumelectron beam device 19 uses an electron beam generated from a triodegun in any convenient manner from a source as indicated at 20. Suchsource can, for example, be of the type described in British Patent No.714,613. It provides a beam which is focused so that it converges at thesite of a resistor 10 as shown in FIG. 3. The resistor 10 is held inchucks 21 which are rotated by suitable driving means not shown. Asshown, the chucks 21 seize the leads of the resistor 10 and hold theresistor 10 in the electron beam.

An air-tight housing 22 forms a chamber 23 which con tains the resistor10, the chucks 21, and mechanisms for moving a succession of resistorsinto engagement by the chucks 21. The electron beam source mounted inconjunction with the housing 22 is arranged to direct the beam into thechamber 23 and impinge it upon the resistors 10 while they are rotatedby the chucks 21. A conventional vacuum pump 24 connected to the chamber23 through a conduit 25 serves to evacuate the chamber 23 and providevacuums down to any desired pressure such as 0.1 micron.

A suitable mechanism for advancing and positioning successive resistors10 is indicated in the chamber 23. The illustrated mechanism iscomprised of a chain belt conveyor 26 moving a plurality of paired yokes27 for engaging the leads 18 of individual resistors 10. The advancingyokes 27 move resistors into position for elevation to the chucks 21.Forks 28 are suitably actuated by means not shown to lift the resistorleads 18 and elevate the resistors 10 from the yokes 27 to the chucks 21for the electron beam treatment and then subsequently to lower theresistors 10 back to the yokes 27. The chucks 21 are connected bycircuitry not shown, so as to be part of the resistance means measuringcircuit described below a! and are also connnected to ground throughcircuitry not shown.

In this invention a surface film type device, and more particularly aresistor, may be helixed by mounting a coated body having a surface filmof suitable resistive or semiconductive material on a pair of yokes 27,advancing the yokes by the chain drive 26 into position for engagementby the forks 28 and moving the resistor up to be gripped by the chucks21. The chucks 21 are then rotated and the electron beam employed toremove a strip of the material to produce the non-conducting path 14.The chamber 23 is evacuated by the suction pump 24 connected to theconduit 25 and this evacuation can also serve to remove the vaporizedproducts from the chamber and thus continue to preserve the electronbeam efiiciency. When the chamber 23 is evacuated by means of the pump24, the electron gun of the electron beam source 20 is energized and theaccelerated electrons emitted into the chamber 23 are focused bymagnetic fields as indicated in FIG. 3, the electron beam impinges onthe rotating surface of the resistor 10 and heats the material of film12 in a localized area to result in its vaporization.

The apparatus can handle any material that vaporizes and the temperaturecan be carried to any desired degree. The width of the beam is readilyadjustable by the focusing means of the electron beam source andaccordingly the width of the nonconducting path 14 is adjustable. Thebeam is made to scan along the length of the resistor 10 by the focusingmeans. In fact, the width of the cut can be adjusted during the helixingof a single unit and the pitch of the path 14 can be adjusted by thespeed of the scan and the rate of rotation.

The conducting track can, for example, be less than 5 mils wide, and thenonconducting path at least as Wide as the conductive path. Three milsis a typical Width for both the conducting and non-conducting tracks. Anentire 4 watt resistor can be not over /2 inch long.

The start of the nonconducting path is accurately controllable as isalso the ending or the termination of the cut of the nonconducting path.These operations are simply achieved by either deflecting the beam ontoor away from the resistor 10 or by focusing or defocusing at theprecisely appropriate instant in time.

The electron beam impinging on the film 12 of resistive orsemiconductive material causes the material to be evaporated and causesthe substrate core in the area of the nonconducting path to be glazed byfusing of the ceramic which takes place under the impingement of thebeam. The surface of the nonconducting path, therefore, ismicroscopically smooth.

The electron beam removes the conductive coating completely vaporizingthe metal film material in an accurate path and striking the exposedsubstrate, heats it momentarily to the fusing point so that just a skinof glazed material is formed. The surface of the non-conducting path,therefore, is free from irregularities and imperfections andmicroscopically smooth. It is also defined by the edges of theconducting track which are extremely straight and parallel and free fromall irregularities. The electron beam, in removing the material alongthe non-conducting path, makes a very shallow cut into the substrate.

This cut is preferably to the depth of less than 0.0005". The lateraledge formed by the conducting track and the shallow groove is gentlyrounded to provided a smooth flare With no sharp edges.

The electron beam source is used to remove the resistive orsemiconductive material in a path by evaporation. The electron beamsource is preferably operated at a high potential of the order of 15 or20 kv. An electron beam current can be produced which causes theresistive or semiconductive material to evaporate under the impingementof the beam of the semiconductive material. This evaporation can becarried on in such a way as to rapidly remove the resistive orsemiconductive material and thus form the non-conducting path. The stepsof this procedure can be programmed into an automatic operation.

The electron beam current may be of the order of 90 microamperes and thebeam power of the order of 2 watts. The current is subjected to a verysensitive grid bias control. The electron gun has the essential parts ofa triode, with a grid bias of 90 v. A representative current out of thegun is 100 microamperes. To cut off the electron beam it is possible toincrease the grid bias to 150 v. This will serve to cut the current downto below 1 microampere.

The means of controlling the electron beam provides a technique for aquick response to a signal for electron beam termination. The formationof the nonconducting path lengthens the resistive path of the conductingtrack and produces progressively an increase in resistance value. It isdesirable to achieve, within a narrow average eviation, an exact finalresistance value for the resistor. This is accomplished by providing aresistance measuring circuit which serves to measure the resistancevalue of the helixed device while the helixing is being carried on. Whenthe final resistance value is achieved by the formation of thenonconducting path, the electron beam is cut off. The time between theresistance determination and this cut off has been cut down to as littleas milliseconds.

In the operation of the apparatus of FIG. 3 for the treatment of theresistive or semiconductive material and the formation of the helixedresistor, the resistor 10 is lifted from the yoke 27 on the forks 28 andplaced in the chucks 21. The chucks 21 seize the resistor leads 18 andholding the resistor 10, rotate it axially. The electron beam isswitched on and focused by adjustment to give the desired impingement onthe material. This beam impingement is similar to that described inapplication Serial No. 807,247. The beam modifies the resistive orsemiconductive material by evaporating a path. The path is formed by theaxial rotation of the resistor and the deflection of the beam along thelength of the resistor to trace a helical path of evaporated materialforming the nonconducting path.

The pressure maintained in the chamber is controllable and may becarried down to an extremely high vacuum such as 0.01 micron. Gasesgenerated within the chamber are removed by evacuation.

The electron beam source is produced according to conditions set forthin application Serial No. 807,247. This electron beam is introduced intothe apparatus 19 through the opening and penetrates into the chamber 23.The electron beam can be focused and can be aimed. Such focusing andaiming is shown in the above-mentioned British Patent No. 714,613. Theresistor body with the resistive or semiconductive material is mountedin the chucks 21 within the chamber 23. The resistor 10 is thensubjected to treatment by the electron beam, as described. After thesteps of treatment, the resistor 10 is removed by release from thechucks 21 into the forks 28 which lower it to a yoke 27. While thistreatment is being carried out, suction pumps attached to the ductscreate a reduced pressure in the chamber and remove gases produced inthe chamber.

As mentioned above, the beam scans the length of the resistor.Individual points on the surface of the resistor are subjected to thebeam for from 3 to 5 milliseconds. The nonconducting path is traced withthe combined scanning and rotation at a rate of the order of 150" perminute to provide a resistor cutting rate of the order of 1 per minute.Such tracing at a rate of at least 100" per minute works very well.

In one aspect of this invention the substrate material making up theceramic core is beryllium oxide. Beryllium oxide is not useful as asubstrate core for a helixed surface film type device produced byconventional methods because of the toxicity of the lay-products fromthe worked beryllia material. Beryllium oxide is a highly desirablesubstrate in this type of resistor, as it assists in providing a highwattage rating for a given hot spot temperature in a resistor. Thedissipation of heat generated in this of resistor is considerablygreater at the two ends. The end caps and leads are factors in providingthis greater heat dissipation. Moreover, the heat is generated mainly inthe helixed portion rather than in the unhelixed end portions.Accordingly, the temperature distribution along the surface is uneven sothat the maximum temperature or hot spot is in the central area with thetemperature falling off at either end. In the beryllium substrate devicethe heat is most rapidly conducted away by the thermal conductivityberyllium substrate material. Accordingly, the resistor stays nearer toa uniform temperature and the rise of temperature in the resistor abovethe ambient is limited or maintained at a minimum.

Ceramic metallic films may provide desirable surface films for this typeof device but present problems in the technique of adjusting theresistance value for an ulti mate useful product. The ceramic metallicmaterial after application to the nonconducting base is tenaciouslyadhesive to the base. The resistance adjusting technique of thisinvention is particularly applicable to this problem. For example, acermet of 65% chromium and 35% silicon monoxide as a surface film can bereadily adjusted to a desired final resistance by the means and methoddescribed herein. Another good resistance material made useful as anadjustable surface film by this invention is molybdenum disilicide.

The above-described invention and its preferred embodiment and the usethereof may be more completely understood by the following example givento illustrate the invention and not intended to be limitative.

EXAMPLE A resistor having a resistance of 1300 ohms was formed bycoating a steatite core with a nickel alloy resistive material, applyinga gold terminal at each end, and attaching tin solder caps. The resistorwas rotated on its axis at a rotational speed of 175 r.p.m. in a vacuumof 0.1 micron. An electron beam having a beam current of microamperesproduced from a beam-producing device under an accelerating voltage of20 kv. with a grid voltage of 75 v. was impinged on the coat of theresistor and swept along the length of the coat in eight seconds. Thebeam vaporized a helical path in the coat and increased the resistanceof the resistor to 2 megohms with A wattage.

A flat substrate carrying a conductive film may also be treated with theelectron beam apparatus in the evacuated chamber. The resistance of theconductive film may be adjusted by vaporizing portions of the film toproduce non-conducting portions. The resultant non-conducting portionshave a smooth surface even though they are made up from the substrateafter removal of the conductive film.

Great flexibility may be obtained in the electrical values of theproduct due to the mobility of the electron beam and the speed of itsoperation on the coated substrate.

Another product of this invention is a surface film type inductor. Asurface inductor is made of a conductive film such as either silver orcopper deposited on an elongated round or bar-like non-conducting baseand formed into a number of closely spaced turns of a conducting trackby a non-conducting path produced in the metallic coat by impingement ofan electron beam. As in the case of the resistor described above, theconductive material removed to provide the non-conducting path isvolatilized and removed from the vacuum furnace by suction.

An advantageous example of an inductance prepared according to thisinvention is the provision of a silver coat on a magnesium oxide core.The silver coat is transformed into a helical conductor by forming anonconducting path with the electron beam. The magnesia core and thesilver conducting track coordinate through their similar coefficients ofexpansion to provide a well matched unit. The silver conducting layer isparticularly adapted to effective cutting by removal with the electronbeam.

In a modification of this invention a resistor body is provided having aceramic substrate carrying a film of resistive or semiconductivematerial and overlain with a layer of ceramic to form a covering. Thisbody can be helixed according to this invention to provide a helicalsurface film resistor. The non-conducting path can be cut from this bodyby the electron beam in the same manner as described above for formingthe non-conducting path in the above described embodiment. The electronbeam vaporizes and removes the overlying layer of ceramic at thenon-conducting path. In this way a ceramic coating can be providedbefore the formation of the helix. Any possibility of contaminationduring the step of applying the coating is thus avoided. A feature ofthe product of this method is the small percentage of exposure of theresistor material during and after the path cutting operation. Only thecut edge of the resistive material is exposed and as the exposed edge isalmost dimensionless, actually it is not sensitive to exposure.

According to an additional optional step in the treatment of theresistor, the resistor may have a protective surface applied bypolymerizing a gas in the chamber for deposit on the resistor after thecutting of the nonconducting path 14 is complete. The chamber may hold agas which when subjected to the electron beam will result in theapplication of a protective layer to the semiconductive material. InFIG. 1 the resistor shown in axial cross section is shown to have aresistive or semiconductible material of the film 12 removed accordingto the process of this invention by the electron beam evaporation andapparatus of FIG. 3 in the non-conducting path 14. The removed portionof the material 10 is the result of removal of the resistive orsemi-conductive material by direct evaporation. The electron beam source20 operates at a sufficiently high potential so that the effect of itsimpingement on the semiconductive material of the film 12 results in anevaporation of the semiconductive material by directing the beam againstthe path 14. This path is removed from the film 12 as indicated by FIGS.1 and 2. To effect this evaporation the resistor 10 is positioned in thechucks within the chamber under a reduced atmosphere to cause theelectron beam to be projected against it without undue loss ofefficiency. Suction pumps evacuate the chamber. The electron beam fromthe beam source strikes the resistor 10 and is focused by previousadjustment to give the desired beam impingement. An electron beam sourceoperates at a sufficiently high potential so that the effect of itsimpingement on the resistive or semiconductive material of film 12results in an evaporation of the semiconductive material by directingthe beam along the path 14. This evaporation is carried on in such a Wayas to rapidly remove portions of the resistive or semiconductivematerial of the film 12 and thus change its shape to the conductingtrack 15.

After the removal of the material 12 from the nonconducting path 14 theresistor 10 is subjected to the next step of resistor manufacture. Thismay include the application of the protective coating.

An important advantage of the electron beam helixing is the smooth edgeof the path when it is cut. This smooth edge results from the melting.In the first place, the profile has a much reduced curvature surface.With the mechanical method a jagged edge is formed. This is undesirablesince it produces sharp points which can be areas of concentration.These areas of concentration are points at which break-down can start.The shape and stability of the conducting path are important. Theelectron beam provides a very even or smooth edge conducting path. (Thisis demonstrated in the control of the magnetic field at the edge of asuperconductor. In a superconductor if the flow along the edge is notdisturbed by the irregularity of the edge, the superconductivephenomenon is preserved. Therefore with the electron beam method ofcutting, a smooth edge is provided so that the conducting path has thedesired straight shape and is stable and preserves thesuperconductivity.)

Among other advantages this invention provides a precise resistor havingresistive values within a close tolerance, including resistors having atolerance of as low as of 1%. The effective narrow cut of thenonconducting path provided by this invention permits a great reductionin size of the metal film resistor for the same value. Correspondingly,there is provided a considerable increase in wattage rating for the samesize unit.

Another advantage is the stabilized temperature coefiicient of theresistance permitted by the good heat dissipation made possible.Moreover, it is possible to match a core and a coat having compatibletemperature coefficients of expansion.

A most important advantage is the faster helixing with fewer rejectswhich makes this invention particularly adaptable to automaticproduction. This is particularly so because of the lower noise in theresultant unit due to the cleaner cut of the non-conducting path and thelack of damage to the substrate.

The electrode beam apparatus as disclosed herein provides a means foradjusting the electrical values of the surface film device veryprecisely. The resistance measuring means, which has a fast response forterminating the grooving upon the attainment of the desired resistance,permits extremely precise control of the product.

The groove or out which makes up the non-conducting turns, is smooth andhas a sharp boundary with the microscopically smooth, straight edge ofthe conductive material. These features provide a surface which is notconducive to entrapment of contaminants. A second advantage of theforegoing grooving technique is provided in the modification whichemploys a ceramic coat over the metal film before the helixing takesplace. With the ceramic coat in place and the non-conducting path cutthrough the coat, the helixing produces a device which needs protectiononly over the extremely thin edge of the metal film. Thus the productionis expedited and simplified.

The flexibility of this means and method will also be appreciated. Itwill operate on any material or combination of materials that vaporize.It is thus effective in making helixed surface film devices withhard-to-handle materials that have other extremely advantageousproperties. The characteristics of the resultant product arecorrespondingly improved. Another facet of the vaporizing in a vacuumaspect of this invention is found in the advantage gained from beingable to process in the vacuum materials otherwise unavailable for usebecause of their toxicity. Beryllia, for example, is generallyunattractive as a material but can be safely used according to thisinvention.

The above-described embodiments and particularly the embodiment treatinga resistive metal alloy film on a steatite Al Si Mag 513 rod, have beenfound to be effective in practice. However, without departing from thespirit of this invention various modifications may be made asexemplified in the modifications indicated above, and therefore it isintended that the invention be limited only by the scope of the appendedclaims.

What is claimed is:

l. A resistor having a cylindrical ceramic member carrying on theoutside of its cylindrical surface a number of closely spaced turns ofhelical electrically conductive coating of metallic electricalresistance material in which the side edges of the turns aremicroscopically smooth and straight, the surface of the ceramic memberbeing glazed in the spaces between turns and recessed below the ceramicsurface on which the turns are coated,

the coating having end terminations between which the 3,136,973 6/1964Randolph 338273 turns are connected. 3,140,379 7/1964 Sohleich et al.21969 2. The resistor of claim 1 in Which the end termina- 3,162,767 12/1964 Di Curcio et a1. 250- 195 tions of the coating are adjacent theopposite ends of OTHER REFERENCES the ceram1c member, each end 1sencircled by a terminal 5 connector, and a stratum of noble metal atthese ends Pfoceedlflgs of 2nd symposlum on Electron Bfiam cooperates inconnecting the terminal connectors to the Processes, On some Aspect ofElectron Beam Evapora' coating ends tions (Thun), March 2425, 1960,Boston, pp. 7093.

Introduction to Electron Beam Technology (Bakish),

References Cited by the Examiner Published by John Wiley and Sons, Inc.,New York- UNITED STATES PATENTS 10 London, 1962 (Chapter 11 1,859,1125/1932 Silverstein 338-330 X 2,671,735 3/1954 Grisdale et a1 338-330 XRICHARD WOOD Prmm Examme" 3,107,179 10/1963 Kohring 117226 V. Y.MAYEWSKY, Assistant Examiner.

1. A RESISTOR HAVING A CYLINDRICAL CERAMIC MEMBER CARRYING ON THEOUTSIDE OF ITS CYLINDRICAL SURFACE A NUMBER OF CLOSELY SPACED TURNS OFHELICAL ELECTRICALLY CONDUCTIVE COATING OF METALLIC ELECTRICALRESISTANCE MATERIAL IN WHICH THE SIDE EDGES OF THE TURNS AREMICROSCOPICALLY SMOOTH AND STRAIGHT, THE SURFACE OF THE CERAMIC MEMBERBEING GLAZED IN THE SPACES BETWEEN TURNS AND RECESSED BELOW THE CERAMICSURFACE ON WHICH THE TURNS ARE COATED,